Method of determining slope of subsurface rock beds



W. G. GREEN METHOD OF DETERMINING SLOPE OF SUBSURFCE ROCK BEDS` FiledNOV. 14, 1932 3 Sheets-Sheet 1 INVENTOR MNMNN mNmSQN WNNQN Wl? BYATTORNEY Dec. 17, 1935. W Q GREEN l 024,921

METHOD OF DETERMINING SLOPE OF SUBSURFACE ROCK BEDS E Filed Nov. 14,1952 .s sheets-sheet 2 fiy. j aw m ma ze 36 d6 .5670 55 IH 7 go Za ENlNvENToR mi/mf me ATTORN EY W. G. GREEN Dec. 17, 1935.

METHOD OF DETERMINING SLOPE OF SUBSURFACE ROCKBEDS Filed Nov. 14, 1932 ssneets-sneet' Patented Dec. 17, 1'935 A UNITED STATESv PATENT 'or-FICEMETHOD F DETERMJNING SLOPE 0F SUBSURFACE ROCK BEDS vWilliam G. Green,Tulsa, Okla. V Application November 14, 1932, Serial No. 642,561

8 Claims. (Cl. 181-05) My invention relates to the art -of surveying.four detectors spaced at 200 feet intervals are subterranean areas fromthe surface of the used would be the difference between 100 and earthfor locating formations indicating the 400 feet or 300 feet. Inlocalities such as the presence of oil and more particularly to a meth-American Gulf Coastal area where the subsurod for determining slope ofsubsurface rock beds. face velocities are comparatively low or ap-4 5 Inattempts to nd the approximate contours proximately 9,000 feet persecond, the latter of subterranean strata and the depth at which -methodis very suitable particularly in View of these formations are located,methods are comthe fact that the inclinationvof slopes around monlyfollowed requiring the use of instruments salt domes of this area iscomparatively great.

10 such as seismographs for detecting sound waves, However, in theMid-Continent area of the 10 sent out by detonation of a shot on or nearthe United States the velocities at which sound earths surface andreflected back by subterrawaves traverse the earth are from 10,100 to11,- nean formations, and oscillographs for recording 000 feet persecond and the slopes usually inthe amplitude of the waves. With oneexcep- Cline only from 0 to 40 feet per 1,000 feet of horition all ofthe methods practiced in the above zontal length of a subsurfacestratum, It will 15 manner, of which I am aware, depend on the thus beevident that the latter method is un# correlation of records obtainedfrom several difreliable for use in high velocity material since ferentpoints, the correlated records indicating v the dierence in time notedbetween the lirst the dierence in slope of a bed lat different andlast-instruments would be so small as to points. If the records arecorrectly correlated make an accurate selection and reading ofthe 50 thecomputation of slope therefrom is a relativerecords extremely diicultand Often impossible.

ly simple matter. However, it is often extremely It is the principalobject of my invention dicult because of interfering waves and lacktherefore, t0 eliminate the. COIlfllSiOi incident t0 of clarity of therecords for various reasons to practicing the foregoing methods and t0PIO- accurately and positively correlate the reiiection vide a methodwhereby slope of subsurface rocks impulses and the drilling of numerouswells at beds can be mOI'B Positively and allaiely degreat expense hasproved that the conclusions termined. v

arrived at by the above method are frequently This object I accomplishprimarily by surveyunreliable. ing as great a continuous length of thereect- 3@ The single exception referredto above is a ing horizon asrequired for definitely indicating method used in low velocity materialwhere a, the contour prole oi'` the rock bed and have difference in timebetween the first and last re-v illustrated SSPS and XeSlllS 0f this'mefJhOd in cording instruments on one set-up of instruthe accompanyingdrawings, wherein: ments is used to determine the slope of the sub- Fig.1 iS a da'ammeti View illlllrating the surface bed between two depthpoints corresuccessive positioning oi detectors and sound spending tothese instruments. In following Sources for surveying an extended areaof a rock this method, the usual practice is to space four bed.seismograph detectors at regular intervals, as Fig. 2 is a diagrammaticView showing three for example, every 200 feet, in a straight lineconsecutive positions of a group of ve seismo- 40 from a shot point orsource of sound (S. SJ. graphs for detecting sound waves originating 40Thus, the four consecutive instruments would from a single shot point.be respectively spaced 200, 400, 600 and 800 feet Figl 3 is a similarView showing the relative from the sound source. From well known lawsdecrease and increase in distances travelled by the depth determined bythe record obtained sound waves reflected respectively from inclined 4@from the rst instrument (200 feet from S. S.) and declined reflectinghorizons. 45 would be that of a vertical line extending e is a copy of aset of correlated records downwardly from a point half way between theobtained by Iollowingrny improved method.

S. S. and first instrument to the reflecting hori- Referring more indetail to the drawings: zon or rock bed. The depth point determined Thelines l and 2 respectively designate the 5U' by a record from the lastinstrument (800 feet earths surface and the surface of a subterranean v0from S. S.) would accordingly be 400 feet h cuin rock bed. A soundsource or shot point, usually zcntaliy from the source oi sound,considering comprising a charge of dynamite, is designated thesubsurface to be level. Consequently, the by SS and seismographdetectors are indicated actual horizontal length of subsurface areasurby the numeral 3. The arrangement and equal yeyed by this method whena single Si. S. and spacing of the detectors in a line with the SS 5a isparticularly illustrated in Fig. 2 wherein the full lines indicate soundwaves emanating from a source of sound and reflected back by the horizonof the rock bed to the group of detectors located in a zone A atstations designated IA to 5A.

The resulting reections detected by the ve seismographs are recorded ona single film strip in the manner indicated on the uppermost strip A ofFig. 4 of the drawings. As above referred to, it will be noted, however,that if the detector at station 5A is located 1,000 feet from SS, thepoint where the sound Wave reaching this detector strikes the rock bedis only 500 feet horizontally from SS or half Way between SS and 5A.Similarly, the reflecting point of the wave reaching the detector atstation IA is feet horizontally from SS so that the total lengthsurveyed of the rock bed surface by the group of five detectors is only400 feet. The records made at the respective stations IA2A3A4A, and 5Aare correspondingly designated in Fig. 4.

Assuming that the surface of the bed inclined 10 feet to every 1,000feet of horizontal length it will be readily evident that the 4 footrise in the length surveyed would result in such a small difference inthe time of arrival of the waves at the rst and last detectors from thenormal time of arrival if the surface were level that the records wouldbe of little or no value since it would be impossible to make accuratecomputations therefrom.

Upon obtaining the first record I, therefore, move the five detectorsrespectively to a zone B including the live stations indicated at IB to5B, the station IB being identical with the station 5A so that the lastrecord on one lm Will be as nearly like the first record of the next lmas is possible.

Another shot is then detonated at SS and the resulting Waves, indicatedby the dot and dash lines in Fig. 2 are recorded by the group of fivedetectors at their stations in zone B. The lin B thus obtained iscorrelated with and mounted' adjacent 4the first lm A as in Fig. 4.'I'his is readily accomplished by comparing the trace 5A on lm A withthe trace IB on film B. Since they were made at the saine station andfrom the same shot point SS they would be' substantially alike althoughthey may vary slightly in intensity due to the fact that the detectorWas previously moved from IA to IB so that differences in ground contactor possibly differences in depth of the holes in which the detector wasplanted may have affected the character of the traces. However thecounted vibrations are the same. 'I'he actual distance surveyed on therock bed surface has now been increased to 800 feet and by repeating theforegoing steps and moving the detectors to stations IC to 5C in zone Cso that the sound waves indicated by dotted lines (Fig. 2) are recordedat this location, the surveyed distance is further increased to 1,200feet.

Considering the inclination of the slope to have continued at the rateof 10 feet per 1,000 feet the last depth point recorded would be only 12feet nearer the earths surface than the rst recorded depth point, anelevational difference so small that accurate computations from therecords thus far obtained Would probably be impossible. The groups ofdetectors are, therefore, successively moved to stations ID to 5D inzone D and IE to 5E in zone E (Fig. 1) -and the previously describedsteps repeated for obtaining record lms D" and E. This process may becontinued indefinitely or until the angle of incidence to the subsurfacerock bed increases to the critical angle of refraction. It is,therefore, possible to survey from one shot point a distance on the rockbed approximately equal to the depth of the bed below the surface of theearth.

In order to carry the steps of surveying still further it now becomesnecessary to move the SS to the point SS (Fig. 1) and it should be notedthat this point is so arranged in a straight line with the previousdetector stations that the Waves from the rst detonated shot at the newsound source and picked up by the iirst detector or 3C adjacent thissource strikes the rock bed at the identical point where the lastrecorded Wave from SS struck the bed thereby producing a record asnearly as possible like the last record theretofore obtained.

Thereupon the process of obtaining records may be continued byconsecutively moving the seismographs to stations ID to 5D, IE to 5Eand. after it becomes impossible, because of too great an angle ofincidence, to any longer obtain reflections of Waves emanating from thepoint SS', the source of sound is moved to the next station or SS(Fig. 1) and the group of detectors is successively placed at stationsIF to 5F and so on. The resulting records are finally arranged in propercorrelation in much the same manner as shown in Fig. 4 and lines 6, l,8, 9, I0, and II may be drawn across consecutive groups of records tofollow a particular reflecting surface indicated by the traces. will, ofcourse, be evident that all of the recording stations and the points ofsound. source should be arranged in a straight line so that the contourof the rock bed surface is surveyed along' a correspondingly straightline.

As will be notedI from the copy of a'n actual record (Fig. 4) the firstportions of the traces are frequently indefinite due to the excessiveinitial amplitude of the waves but the latter portions are clear and byhaving a number of records showing the results of surveying an extensivearea it is possible to determine With accuracy the general slope of asubsurface rock bed.

The procedure of calculating the depths of a bed from records divided bytransverse lines I3 into equal time intervals is well known to thoseskilled in-the art. For computation purposes it is desirable to assumethe rock bed to be level as shown in Fig. 2, and it will be clear fromthis figure that the depth obtained from the rst instrument reflectionat Station IA is approximately one-half the distance from SS to the rockbed and back to the deector at this station or d/2. Since the record ofthe reflecting Wave takes nothing into account except the time consumedin traversing the path from SS to the rock bed and back to station IA, dis recorded as a function of the time T of the reflection.

By taking the station ASS as a center and swinging an arc I4 from therock bed it is found that the length of path traversed by the wavesincreases in proportionto the spacing of the detectors from the soundsource. Thusl the increase in the path d/2 as the distance of thedetector from SS increases is designated delta d/2 which represents adelta time, or normal delta T for the level subsurface assumed. Thisnormal delta T increases as the surface of a rock bed declines, andincreases in proportion to the inclination of the surface as illustratedin Fig. 3.

If, on computing the recorded reflections of an instrument set-up adelta time is observed which differs from the expected delta T, thisdifference, after correcting for elevations, weathering, etc., would bedue-to the difference in time required for the waves to travel theirpaths and would designate the slope of the bed. Therefore, thedifference between the normal expected delta T and the observed delta Tfor any particular distance from the S. S. will be the result of a slopein the surveyed portion of the surface of the rock bed.

For the purposes of simplifying and expediting the computation ofrecords obtained by the method above described a graphical solution hasbeen worked out whereby the amount and direction of a slope may beaccurately ascertained. Formulas showing the relation of the variousquantities may also be derived to simplify the process of calculatingthe amount of slope.

Calculation of the slope of a subsurface bed from a record as shown inFig. 4 is thus comparatively simple. tively loose or unconsolidatedlayer of weathered material extending downwardly from the surface of theearth at varying distances trans-` mits sound at a very low velocity itis necessary to first find the base of this layer', designated i5, Fig.3, and to thereafter make the neces- .sary corrections on the records sothat the final calculation will be as nearlyaccurate as possible. Thedepth of the weathered layer at each detector station may be determinedby any well known method. y

In practice I compute this depth by the refraction method from the firstarrival time of the records, this time being indicated by a sudden breakin a trace near its beginning as shown at I5 on the fourth record stripof Fig. 4. The indication, in the trace, of the exact instant at whichthe shot was detonated would be to the left of the break I5 and is notshown in this figure. j

From the foregoing it will be apparent` that in practicing thepresentinvention, a positive correlation of various records and an accurate andreliable calculation of depths is made possible. Since the difference indepth from one point to another can be accurately determined and thisprocess continued indefinitely it is possible with my method to obtain acomplete profile of the surveyed slope of a subsurface -rock bed. Inorder to check the computations made on the records obtained by theprocess above de.

scribed the same procedure may be followed but in the reverse direction,thus producing an effect opposite Yto the former, i. e., an inclineindicated by the latter should correspond to a decline disclosed by theformer procedure and vice versa.

What I claim and desire to secure by Letters Patent is:

i. The method of profiling a subsurface formation including detonatingan explosive charge at one point, recording the refiection of soundwaves set up by the detonation and reflected from said formation at aseries of spaced detecting' points, advancing such series of ldetectingpoints in definite relation to the first series of detecting points sothat the rst one of the detecting points .of the second series is commonto a detecting point of the first series,

detonating a second explosive charge at said However, because therelarst point of detonation, recording the sound waves set up by thesecond detonation and reflected from said formation to said secondseries of detecting points, and correlatingsaid records relatively torecordings from said common detect- 5 ing points wherefrom a continuousprole of said f formation is determined.

dence by a different path to provide a plurality of records wherebyprofile of said `formation is 20 determined by correlation of saidrecords. l 3. The method of profiling a subsurface forl mation includinggenerating sound waves at a4 point above said formation, recordingreception of said waves reflected from said formation at a plurality ofaligned points spaced from saidgenerating point to determinefpoints ofincidence `of said waves with said formation, generating similarN wavesat a point spaced from said -first generating point and common to one ofsaid points of reception, and recording reception 'of the wavesreflected from saidv formation at a plurality of spaced points from thesecond genera'ting point, the first of 'which being wavesl reflectedthrough the last point of incidence by a different path to provide aplurality of records whereby a continuousprofile of said formation isdetermined by correlation of said records.

4. The method of profiling a subsurface formation includingprogressively vproducing at l0 spaced sources successive sets of seismicwaves,` and recording reception of the sets of waves reflected from saidformation at such regularly spaced distance relation to effect receptionof one set of reiiected waves from one source through an incidence pointon the formation common to the incidence point of a vrecorded set ofre-` flected waves from an adjacent source.

5. The method of profiling ya subsurface formation including producingsets of sound waves at aI common source above said formation, recordingreception of the sets of waves reflected from progressively spacedincidence points on said formation and to the approach of a pointproducing a critical incidence angle, producing similar sets of soundwaves at a point common to. one of said recording points, and recordingreception of the sets of waves reflected by the formation'from saidsecond source to establish a` series of records each having a recordingcommon to a recording on another whereby the records are correlated todetermine profile ofsaid formation. .f

6. The method of profiling a subsurface formation including producingsets of sound waves at a source above said formation, recordingreception of the sets of waves reflected from progressively spacedincidence points on vsaid formation and to the approach of a pointproducing a critical incidence angle, producing similar sets of soundWaves at a source spaced from said`first source and common to one of'said points of reception, and recording v:recep\ tion of the sets ofWaves reflected by the formation from said second-source in such spaceddis-l tance relation to effect reception of one set of reflected wavesfrom the second source through an incidence point on the formationcommon to an incidence point of a reflected wave recorded from the firstsource.

7. The method of profiling a subsurface formation including detonatingan explosive charge at one point, recording the reection of sound wavesset up by the detonation and reected from said formation at a series ofspaced detecting points, progressively advancing said series ofdetecting points in definite relation to the preceding series ofdetecting points so that one of the detecting points of a succeedingseries is common to a detecting point of the preceding series,detonating an explosive charge at said first point of detonation foreach series of detecting points to provide a plurality of records eachhaving a recording common to a recording from a preceding record, andcorrelating said records relatively to said common recordings wherefroma continuous prole of said formation is determined.

8. The method of proling a subsurface formation including generatingsound waves at a point above said formation', recording reception ofsaid sound waves r'eected from said formation at a plurality of spacedpoints, generating a similar set of waves at the point of reception ofone of said reflected waves from the rst generating point, and recordingreception of the waves reilected by( the formation from said secondgenerating point at a plurality of spaced points including one of therst points of reception to provide a plurality of records each 15WILLIAM G. GREEN. 20

